var producten = (from p in Premy.All()
     join pr in Producten.All() on p.dekking equals pr.ID
     where p.kilometragemax >= 10000 &&
           p.CCmin < 3000 &&
           p.CCmax >= 3000 &&
           p.leeftijdmax >= DateTime.Today.Subtract(car.datumEersteToelating).TotalDays / 365
     group p by new { pr.ID, pr.Naam, pr.ShortDesc, pr.LongDesc } into d
     select new
     {
         ID = d.Key.ID,
         Dekking = d.Key.Naam,
         ShortDesc = d.Key.ShortDesc,
         LongDesc = d.Key.LongDesc,
         PrijsAlgemeen = d.Min(x => x.premie),
         PrijsAlgemeenMaand = d.Min(x => x.premie),
         PrijsMerkdealerMaand = d.Min(x => x.premie),
         PrijsMerkdealer = d.Min(x => x.premie)
     }).ToList();

List<QuotePremies> producten = (from p in Premy.All()
     join pr in Producten.All() on p.dekking equals pr.ID
     where p.kilometragemax >= 10000 &&
           p.CCmin < 3000 &&
           p.CCmax >= 3000 &&
           p.leeftijdmax >= DateTime.Today.Subtract(car.datumEersteToelating).TotalDays / 365
     group p by new { pr.ID, pr.Naam, pr.ShortDesc, pr.LongDesc } into d
     select new QuotePremies
     {
         ID = d.Key.ID,
         Dekking = d.Key.Naam,
         ShortDesc = d.Key.ShortDesc,
         LongDesc = d.Key.LongDesc,
         PrijsAlgemeen = d.Min(x => x.premie),
         PrijsAlgemeenMaand = d.Min(x => x.premie),
         PrijsMerkdealerMaand = d.Min(x => x.premie),
         PrijsMerkdealer = d.Min(x => x.premie)
     }).ToList();

public class QuotePremies
{
    public byte ID { get; set; }
    public string Dekking { get; set; }
    public string ShortDesc { get; set; }

    public string LongDesc { get; set; }
    public decimal PrijsAlgemeen { get; set; }
    public decimal PrijsAlgemeenMaand { get; set; }
    public decimal PrijsMerkdealer { get; set; }
    public decimal PrijsMerkdealerMaand { get; set; }
}

PrijsAlgemeen = Convert.ToDecimal(d.Min(x => x.premie))

public static class Extensions
{
    // extends IEnumerable to allow conversion to a custom type
    public static TCollection ToMyCustomCollection<TCollection, T>(this IEnumerable<T> ienum)
           where TCollection : IList<T>, new()
    {
        // create our new custom type to populate and return
        TCollection collection = new TCollection();

        // iterate over the enumeration
        foreach (var item in ienum)
        {
            // add to our collection
            collection.Add((T)item);
        }

        return collection;
    }
}